SUMMARY: Parathyroid hormone (PTH) is a central regulator of calcium and phosphate homeostasis and thereby essential for bone mineralization. In primary hyperparathyroidism parathyroid glands (PTGs) produce increased amounts of PTH despite normal or high serum calcium levels. The molecular mechanisms that normally control transcriptional regulation of PTH in parathyroid cells are poorly defined. Moreover, PTH productionisalsoinfluencedbymechanismsthatcontrolitsproteinstabilityorsecretionfromcells.Identifying transcriptionalorposttranscriptionalregulatorsofPTHproductionisimportantforourunderstandingofthebasic mechanisms that control PTH production. Here, we have identified the BTB-domain-containing nuclear proteinKCTD1asanovelkeyregulatorofPTHproductionandweproposethatlackofKCTD1inPTGs leads to primary hyperparathyroidism through loss of KCTD1-mediated transcriptional repression of PTH.WegeneratedKCTD1lacZreportermiceandKCTD1-/-miceandshowthatKCTD1isstronglyexpressedin PTGs and that mice lacking KCTD1 have highly increased active PTH serum levels and hypercalcemia. Moreover,KCTD1isexpressedinhumanparathyroidtissueaswell.KCTD1-/-micedevelopbonemineralization abnormalities,astheyareseeninpatientswithhyperparathyroidism.Notably,someoftheboneabnormalities inKCTD1-/-micephenocopybonedefectsseeninpatientswithScalp-Ear-Nipplesyndromeinwhichweidentified KCTD1 missense mutations. To address the question if the abnormalities observed in KCTD1-/- mice are a consequenceoflossofKCTD1specificallyinthePTGsorwhetherfunctionsofKCTD1inotherorganscontribute to the observed phenotype as well, we have generated mice that lack KCTD1 only in PTGs and express a fluorescent reporter allele, allowing us to dissect PTGs from these mice (PTH-Cre+KCTD1fl/flEYFP+ mice). We willcomparethephenotypesinthesemicewiththoseobservedinKCTD1-/-miceandtherebydetermineifthe observed hyperparathyroidism and bone defects are a consequence of KCTD1 deficiency exclusively in the PTGs. KCTD1 can function as a transcriptional repressor and as an inhibitor of canonical Wnt/?-catenin signaling. To determine how KCTD1 regulates PTH production we will test in dissected PTGs of PTH- Cre+KCTD1fl/flEYFP+ mice whether KCTD1 regulates PTH production through transcriptional repression or througheffectsonproteinstabilityorcellularsecretionofPTHandwhetheritaffectssensitivityofPTHproduction inresponsetochangesincalcium.WewillinvestigatewhethercanonicalWnt/?-cateninsignalingisincreased inPTGsthatlackKCTD1andwhether?-catenininhibitorscanrescuetheincreasedPTHproduction.RNA-Seq will identify downstream targets of KCTD1 in PTGs. Thus, the scientific premise is high given our extensive preliminarydataandtheavailabilityofthePTG-specificKCTD1KOmice,andtheproposedexperimentshave animportantclinicalsignificanceforourunderstandingofpathomechanismsinvolvedinhyperparathyroidism.